I just measured my F4S to kinda give you an idea of how big it is. It is 6-1/2" wide, 5-1/4" tall including the prism, and 2-1/4" thick at the thickest point, which is the grip.
That doesn't sound too bad. With the MB-20 it'll be practically the same size as my 5DmkIIs. I think I'm at a middle of the road point between the two cameras. I like the sound of both but I suppose I'll buy one and if I don't like it, sell it and buy the other.